Ultrasound Physics |

US scans have been used to image the human body for at least 50 years and is currently one of the most important, widely used, and versatile imaging modalities in medicine.

US images are tomographic and are acquired at "real-time" rates.

The technology is relatively inexpensive and portable, especially when compared with modalities such as magnetic resonance and computed tomography

it poses no known risks to the patient.

Creating an ultrasound image is done in three steps - producing a sound wave, receiving echoes, and interpreting those echoes.

Producing a sound wave

Ultrasound waves are produced by a transducer. A transducer is a device that takes power from one source and converts the energy into another form eg electricity into sound waves. The sound waves begin with the mechanical movement (oscillations) of a crystal that has been excited by electrical pulses, this is called the piezoelectric effect.

The sound waves are emitted from the crystal similar to sound waves being emitted from a loud speaker. The frequencies emitted are in the range of (2- 15MHz) and are unable to be heard by the human ear. Several crystals are arranged together to form a transducer. It is from the transducer that sound waves propagate through tissue to be reflected and returned as echoes back to the transducer.

Precise electrical pulses from the ultrasound machine make the transducer create sound waves at the desired frequency. The sound is focused either by the shape of the transducer (Curved, Linear, Sector) or a set of control pulses from the ultrasound machine. This focusing produces the desired shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth.

On the face of the transducer a rubber material enables the sound to be transmitted efficiently into the body. This rubber coating is required for impedance matching and allows good energy transfer from transducer to patient a vice versa. To help with the transmission of sound waves a water based gel is placed between the patient's skin and the probe.The gel establishes good acoustic contact with the body, since air is a very good acoustic reflector.

Receiving the echoes

The image is formed by the reverse of the process used to create the sound waves. The returning echoes to the transducer are converted by the crystals into electrical signals and are then processed to form the image.

Forming the image

To form the image ultrasound machine needs to determine the direction of the echo, how strong the echo was and how long it took the echo to be received from when the sound was transmitted. Once the ultrasound scanner determines these three things, it can locate which pixel in the image to light up and to what intensity.

Sound in the body

When a sound wave encounters a material with a different density (acoustic impedance), part of the sound wave is reflected back to the transducer and is detected as an echo. The time it takes for the echo to travel back to the transducer is measured and used to calculate the depth of the tissue interface causing the echo. The greater the difference between acoustic impedances, the larger the echo is.

Highly reflective interfaces give rise to a strong echo which is represented on the screen as a bright spot, whilst the opposite is true of weak reflective interfaces. Areas without acoustic interfaces such as the lumen of vessels and other cavities containing liquid (blood, bile, ascites or urine) give no reflection and no spot on the screen ie a black space on the monitor. If the waves hits gases or solids the density difference is so great that most of the acoustic energy is reflected and it becomes impossible to see deeper.

The speed of sound is different in different materials, and is dependent on the acoustical impedance of the material. However, the ultrasound scanner assumes that the acoustic velocity is constant at 1540 m/s. An effect of this assumption is that in a real body with non-uniform tissues, the beam becomes de-focused and image resolution is reduced.

The formula for the velocity of sound is (velocity = frequency x wavelength). The frequencies used for medical imaging are generally in the range of 2 to 15 MHz. Higher frequencies have a smaller wavelength (as can be seen from the formula for velocity of sound), and can be used to make images with smaller details. However, the attenuation of the sound wave is increased at higher frequencies, so in order to have better penetration of deeper tissues, a lower frequency 3-5 MHz is used. Seeing deep structures in the body with ultrasound is very difficult as some acoustic energy is lost every time an echo is formed, but most of it is lost from acoustic absorption.

Apparatus

Scanner typesFor abdominal ultrasound curved type scanners are used as the best compromise of two other standard type probes the linear and the sector scanner. Linear - the linear array scanners produce sound waves parrallel to each other and produces a rectangular image. The width of the image and number of scan lines are the same at all tissue levels. This has the advantage of good near field resolution. Often used with high frequencies ie 7MHz. Can be used for viewing surface texture of the liver. There disadvantage is artifacts when applied to a curved part of the body creating air gaps between skin and transducer.Sector/Vector - Produces a fan like image that is narrow near the transducer and increase in width with deeper penetration. It is useful when scanning between the ribs as it fits in the intercostal space. The disadvantge is poor near field resolution.

Curved - Often with frequencies of 2 - 5 MHz (to allow for a range of patients from obese to slender). It is a compromise of the Linear and Sector scanners. The density of the scan lines decreases with increasing distance from the transducer. Can be difficult to use in curved regions of the body eg. the spleen behind the left costal margin.

Controls

Ultrasound machines have a large array of options and features. The basic controls that you need to familiarize yourself in the early stages of learning areTrackball - used for moving objects on the monitor (similar to using a mouse on the PC), it is used in conjunction with measuring, annotating, moving Res/Dopler boxes to the desired location. It has kidney buttons either side which are used to select functions (the same as clicking buttons on a mouse for the PC).Freeze - This allows the image to be held (frozen) on the screen. While the image is frozen measurements can then be taken and organ annotations can be applied to the image before saving it. Res or Zoom - This will allow magnification of areas of the ultrasound picture. Looking at Res/Zoomed areas of interest has the advantage of a more detailed view with the drawback of less anatomy visible to guide your movements.Caliper - This is used to measure a distance (eg kidney length). It is used by selecting a starting spotby pressing a kidney key and using the trackball to measure to a second mark. The distance between the two marks will then be displayed on screen measured in cm. This can be used with other functions such as Res/Freeze.

Gain - This function is very similar to a brightness control. The echo signal returning to the body is converted into an electronic signal by the transducer. This electronic signal has to be amplified to produce images on the monitor. This signal amplification is called Gain and will regulate the strength of the echo’s being received.

Time Gain - Is an adjustment for the sensitivity at each depth to allow compensation for signal loss from deeper in the tissue. This can be set so that organs such as the liver will have uniform brightness at all depths. It is a series of multiple sliders so you can set the time gain differently for each depth.

Artefacts

ReverberationReverberation is the persistence of sound in a particular space after the original sound is removed